This is already available in `light' auto-hinting mode since a very long time, however, it was poorly documented, namely, to use `lsb_delta' and `rsb_delta' to shift the glyph by a fractional amount before rendering – the reference only gave an example how to use the two variables for computing integer advance widths.

The `ftdiff' demo programs has been updated in git to demonstrate sub-pixel hinting in `light' auto-hinting mode.